The persistent vector was a flat Scheme vector with copy-on-write: every conj
copied the whole backing array, so building an n-element vector was O(n^2). On the
collections bench that's vec-sum building 7500 elements with 227MB of copies, 90%
of the bench's allocation.
Replace it with Clojure's PersistentVector — a 32-way trie plus a trailing tail
chunk. conj appends to the tail and, when it fills, path-copies it into the trie,
so conj is O(1) amortized and a linear build is O(n). nth/assoc/pop are
O(log32 n). make-pvec (build a trie from a flat vector) and pvec-v (materialize
back) stay as compatibility shims, so the ~14 callers that read the backing array
— all one-shot conversions in =, hash, seq, meta-copy, transients, the reader —
are untouched; only this file's internals change.
vec-sum 70ms/227MB -> 1ms/3MB; collections 10.4x -> 4.0x vs JVM, under the 5x
target. 5 new corpus rows plus boundary stress (level transitions at 32 and 1024,
pop-collapse, assoc at every index) cover the trie.
Co-authored-by: Yogthos <yogthos@gmail.com>
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